This invention relates to turbofan engines and particularly to mufflers for suppressing noise and utilizing the plug cone in the engine exhaust stream.
U.S. Pat. No. 4,100,993 granted to E. Feder on July 18, 1978 entitled ACOUSTIC LINER and assigned to the same assignee as this patent application discloses a similar acoustic liner where slots are in communication with a cavity for suppressing sound. As shown therein slots having depending lips are disposed transverse to the direction of flow of the grazing flow. The slots and cooperating lips form a space or volume for supporting a column of air for obtaining a desired mass reactance.
I have found that by the proper orientation of the slots relative to the grazing flow I have unexpectedly realized at least 4--5 decibel reduction in the noise level when in the frequency range of about 400 Hertz. In the exhaust plug muffler for a jet engine to which this invention is applied, the plug is hollow, closed at both its upstream end and downstream end by barriers. Although the flow in the jet engine is essentially annular and the plug is mounted coaxially to the annular flow and in the quiescent portion, it is susceptible of seeing some flow. The flow that migrates through the slots tends to flow internally of the plug and out of other slots. Any flow into and out of the slots is deleterious to sound suppression. Thus, the plug is designed, according to this invention, to eliminate or minimize any flow into and out of the slots. To this end, baffles radiating radially from the inner diameter to the plug center line and extending the axial dimension serve to alleviate this adverse flow problem.